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The movement of plants towards light is called 'phototropism'. 'Photo' means light, whereas tropism means 'to move'. Plants grow by plant cells getting longer, or splitting into new cells. The cells on the side away from the light grow much faster than those on the sunlight side. This causes the plant stems to bend towards the light. Phototropism allows the plant leaves to get the full benefit of the sunlight energy.

The green chemical in plant leaves is called chlorophyll (....."Clo - ro - fill".....) changes sunlight into energy and sugar. The plant in the dark cupboard loses its green colour because sunlight helps to make chlorophyll.
Without chlorophyll plant growth stops as there is no more energy or sugars being produced.